Very Low IF receivers are widely used for slot-based radio communications and, in particular Time Division Multiple Access (‘TDMA’) protocols such as the Global System for Mobile communications (‘GSM’), Digital Enhanced Cordless Telecommunications (‘DECT’) and Enhanced Data for GSM Evolution (‘EDGE’), in the General Packet Radio Service (GPRS), an extension to the GSM standard that provides higher speed access. These receivers need to reject interferer signals that fall as image frequencies on or very close to the wanted signal when the wanted signal is translated to the desired intermediate frequency. This is achieved through image cancelling mixers or poly-phase filters at low frequency. The success of these image cancelling techniques depends upon the balance achieved in terms of phase and gain of the I (in phase) and Q (quadrature phase) paths in the receiver. Patent specifications U.S. Pat. No. 6,597,748 and EP 1 058 378 describe receivers of this kind.
The balance of I and Q in the receiver may be achieved by precision analogue design and by compensation in the form of a digital equaliser. This adds cost in terms of yield, software overhead, manufacturing overhead and extra hardware. Moreover, the image rejection varies with temperature and frequency band.
The image channel is often one of the adjacent frequency channels or alternate adjacent channels to the wanted one. The other adjacent or alternate adjacent channel is not an image frequency and can be rejected adequately without the need for such accurate quadrature balance. Instead it is easily rejected by standard filter topologies. Selectivity is especially a problem for alternate channels on the ‘low side’ IF, that is to say where the local oscillator (‘LO’) frequency used to convert the carrier frequency down to IF is smaller than the carrier frequency, so that the IF is positive, regarding interferers at twice channel spacing (400 kHz in the example shown in FIG. 2 of the accompanying drawings) since the receiver treats them like an image and part of the interferer spectrum falls in band.
Patent specification US2002/0183030 describes radio receiver and transmitter apparatus that includes Local Oscillator (LO) frequencies LowLO and HighLO selectable based on the frequency band of operation of the mobile communication device in the transmitter, whereas in the receiver the LO inputs LowLO and HighLO are connected to different receiver paths via a sub-harmonic mixer.
The specifications for low IF radio receivers include stringent requirements regarding the rejection of interferers and there is a need to meet or surpass these specifications at minimal cost.